jmphash : Jump consistent hashing

physh : Physical hashtable

Hashtables indexing OCaml values by their physical indentities. A
proof-of-concept, playing with the GC in tricky ways.

Its main purpose is to efficiently observe sharing, detect cycles, etc, in
arbitrary OCaml values without having to stop and stay out of the OCaml
runtime.

Can be used to experiment and learn about the GC but do expect bugs and don't
expect any kind of compatibility with future OCaml versions.
(Would be nice to have proper upstream support for such feature though!)